There is a known conventional optimum design method for antennas having structures in which metal patches are arranged on an antenna element plane, which uses an antenna optimum design method for designing structures of microstrip antennas using a genetic algorithm shown in non-patent document 1.
This conventional antenna optimum design method will be described with reference to FIGS. 1A and 1B.
As shown in FIG. 1(a), the antenna includes a ground plane 100a with a metal surface, an antenna element plane 100b formed in parallel with the ground plane 100a and provided with a metal patch on a surface thereof, a feed point 100c connected to the ground plane 100a for feeding the metal patch on the antenna element plane 100b, and a short-circuit element 100d for short-circuiting the metal patch on the antenna element plane 100b and the metal surface on the ground plane 100a. The space between the ground plane 100a and the antenna element plane 100b is filled with air or dielectric.
Antennas according to the preset invention include every antenna having a structure in which metal patches (meander lines) are arranged on an antenna element plane, such as meander-line antennas, planar inverted-F antennas, planar inverted-L antennas, and small multiband antennas.
As shown in FIG. 1(b), the metal patch on the antenna element plane 100b is divided into a lattice of equal-sized blocks of a rectangular shape (including a square shape. The same below.). Next, a one-bit chromosome is assigned to each block. Then, it is determined whether to remove a metal patch in each block or not. For example, as shown in FIG. 1(b), a metal patch in a block whose chromosome is “0” is removed, while a metal patch in a block whose chromosome is “1” is not removed.
Accordingly, the conventional antenna optimum design method is configured to search for optimum chromosomes for making up an optimum antenna from among antennas of random shapes by a genetic algorithm using a given evaluation function.
In FIG. 1(b), the number of a metal patch is a unit number given to the metal patch.
There is another known conventional antenna optimum design method which uses a genetic algorithm as shown in patent document 1.
(Patent Document 1) Japanese Published Unexamined Application No. 2001-251134
(Non-patent Document 1) Tamami Maruyama, Keizo Cho, “Analysis of Design Method by GA for Multifrequency Shared Antenna”, Society Conference of the Institute of Electronics, Information and Communication Engineers, 2003, B-1-198
(Non-patent Document 2) Masanori Ohira, Hiroyuki Deguchi, Mikio Tuzi, Hiroshi Kanizawa, “Analysis of Rectangular Waveguide with Binding Window of Random Shape”, MW2003-212, pp. 25-30, 2003
However, a method using the conventional antenna optimum design method shown in the non-patent document 1 generates a structure in which two metal patches are in contact only at a vertex as shown in FIG. 1(b) (e.g., metal patches of unit numbers “B1” and “B2”, and metal patches of unit numbers “B3” and “b4”).
Antennas including such a structure generally have problems as described below:    (A) An extremely narrow width of a meander line at a contact between two metal patches makes a usable frequency bandwidth significantly narrow;    (B) When chromosomes are constituted by random numbers, a meander line is unlikely to have a continuous shape, and it takes time to calculate an optimum solution in a genetic algorithm;    (C) Manufacture using a drill or the like is impossible; and    (D) Characteristic degradation due to manufacturing error is likely to occur.
Since such a structure is generated more frequently as the number of divisions of an antenna element plane constituting an antenna is increased, there is a problem that it is almost impossible to completely eliminate such a structure no matter how many times optimization by the genetic algorithm is repeated.
Also, the conventional antenna optimum design method shown in the patent document 1 did not mention an antenna optimizing method for designing meander-line antenna structures.